The following relates generally to a light source consisting of one or more arrays of light emitting diodes (LEDs), incorporating multiple LED chips of different colors or wavelengths, and in particular to a light source that uses a multi-layer circuit board, in order to improve the color mixing, color rendering and color temperature control of the light source.
In order to provide light of a specific color, or white light of a specific color temperature, with high brightness, it is normally necessary to use multiple LEDs or LED chips of different colors or wavelengths, so that the combined light output of the multiple LEDs has the desired spectral characteristics. A high quality light spot created from a light source is characterized by its spatially uniform chromaticity, or color temperature when characterizing white light. Achieving a spatially uniform light spot has not been challenging to do with traditional light sources such as tungsten bulbs and HMI (hydrargyrum medium-arc iodide) lamps since it is easier to produce a uniform light spot from a uniform source rather than a non-uniform one. However, producing a uniform light spot is more of a challenge for modern light sources that rely on the mixing of light from multiple LED or LED chips, which typically require additional light shaping and/or mixing optics such as diffusers to achieve good color uniformity.
In order to have a uniform spot being projected to an object, from a light source comprising an array of LED chips of multiple colors or wavelengths, each LED chip should be surrounded by LED chips of different colors in a two-dimensional plane, with the LED chips kept as close together as possible. Put another way, the LED chips of multiple colors should be intermixed or interspersed within the LED array, to improve the uniformity of the combined light output of the LED array.
In the case of high-quality white light sources, such as are used in the fields of photography, video, cinematography, and architectural lighting, it is also necessary for the light source to have a high Color Rendering Index (CRI), as well as a well-controlled color temperature (CCT). In addition, the ability to provide tunable or adjustable color temperature (CCT) is highly desirable, with high CRI at all color temperature settings. These requirements and objectives impose restrictions on the selection of LED colors and wavelengths, and the quantities used of each type of LED chip used in the LED array.
For efficient driving of the LED chips of an LED array, it is desirable to connect multiple LED chips together to form one or more series strings, with each series string driven by a controlled current source. In order to provide adjustability or tunability of the combined light output of the LED array, it is necessary to independently drive different subsets of the LED chips used in the array, as separate LED strings. If the LED array comprises LED chips of multiple colors or wavelengths, typical practice would be to connect the LEDs of like color or wavelength into one or more independently-driven strings. This allows the relative brightness of each color to be independently adjusted.
The substrate for mounting multiple LED chips to form one or more LED arrays must provide for electrical connectivity between the LED chips, or various subsets of the LED chips. In addition, electrical isolation or insulation must be provided between the different subsets of LED chips, and also to allow LED chips to be connected in series strings. The substrate must also provide good thermal conductivity, to efficiently transfer heat away from the LED chips. This is extremely important in the case of LED arrays that use large numbers of LED chips, packed closely together to provide an intense, high-brightness light source. Conventional epoxy-glass printed circuit boards (PCBs) and even ceramic substrates are not suitable because of poor thermal conductivity. A substrate having offering both good electrical conduction and insulation where desired, as well as good thermal conductivity, is highly preferred. Metal or semiconductor wafers with an electrically-insulating layer built on the top of the wafer, represent a good choice as the basis of a mounting substrate for LED arrays.
In an LED array comprising LED chips of multiple colors, with independent control of the multiple colors, the electrical connections of the LED chips into strings is made easier by clustering LED chips of like color. However, as stated above, this leads to poor color mixing, and poor spatial uniformity of the spot beam produced by the LED array light source. Color mixing and uniformity are both improved by spatially intermixing the LED chips of differing colors. In order to achieve good color mixing and uniformity, the LED array's mounting substrate should support the electrical connection of LED chips that are not adjacent to each other, and whose locations on the substrate may be fairly widely separated.